ananyo writes: "People seem to have more than one way to work themselves into a panic. Contrary to a long-standing assumption of neuroscientists, humans can experience fear even when they lack the brain structure widely regarded as the brain's 'fear centre'.Many studies have shown that the amygdala, a small, almond-shaped structure located deep inside the brain, is crucial for the fear response.Now researchers at the University of Iowa have found that in certain situations the fear response may occur even in people who do not have a working amygdala.One situation in which the amygdala triggers fear and panic attacks is when it detects unusually high concentrations of carbon dioxide — a sign of possible suffocation — by sensing increased acidity in the blood. The researchers asked patients with damaged amygdalas and healthy volunteers to inhale 35% carbon dioxide and, to their surprise, found the brain-damaged patients became even more fearful and panicky than the control group."